and of certain Liquids on the Surface of Water. 381 



mon surface of the two liquids must be very small, and that con- 

 sequently a thin film of oil must spread over the whole surface 

 of the water, while a second drop would assume the lenticular 

 form. I give Professor Challis's own words, since a similar 

 theory was published by DuBois-Reymond 22 in 1858. He says: — 

 "The angle of actual contact between two fluids is determined 

 by the hydrostatical equilibrium resulting from the molecular 

 attractions of the two substances, the fluids being treated as in- 

 compressible. It thence appeared that this is an exceedingly 

 small angle in cases in which the bodies in contact are not of 

 very different specific gravities. Hence in the instance before 

 us, the angle of contact (that is, the angle which the surface of 

 contact of the oil and water makes with the upper free surface 

 of the oil) is very small. But since the drop is convex both at 

 its upper and under surfaces, this is apparently an angle of con- 

 siderable magnitude. In fact the theoretical angle of contact, 

 or that which the upper surface of the oil makes with an imagi- 

 nary surface drawn parallel to its under surface, and just beyond 

 the sphere of the molecular action of the water, would be found 

 by calculation to be of sensible magnitude. Consequently, that 

 the angle of actual contact may be exceedingly small, the por- 

 tion of the upper surface of the oil that lies within the sphere of 

 the molecular action of the water must undergo a flexure near 

 the visible periphery of the drop. Now in fulfilling this condi- 

 tion it seems probable that a very thin film of the oil spreads 

 over the whole water surface (as there is no force to counteract), 

 and gives rise at the same time to the visible spreading of the 

 first drop. The film itself, being of less thickness than the 

 radius of the sphere of the molecular action of the water, will 

 not be perceptible to the senses. Such, a circumstance having 

 happened to the drop that first comes in contact with the water 

 will prevent any that succeed from being similarly affected." 



15. It is remarkable that Professor Challis should have 

 thought it theoretically probable that a thin [invisible] film of 

 the oil spreads over the whole surface of the water and assists the 

 visible [and slower] spreading of the first drop, because in some 

 oils (such as oil of cinnamon) this thin film is visible. On 

 depositing a drop of this oil on the surface of clean water in 

 a clean glass 3i inches in diameter, an exceedingly thin but 

 visible film is instantly drawn over the whole surface of the 

 water, while the denser film, which does not cover half the sur- 

 face, spreads much more slowly. In the case of freshly distilled 

 oil of coriander (the very remarkable cohesion-figures of which 

 are given in one of my papers 23 ), the first thin film is accompa- 



22 Pogg. Ann. vol. civ. p. 193. 



23 Phil. Mag. S. 4. vol. xxxiii. plate iv., June 1867. 



